1. Field of the Invention
The present invention relates to an automatic focusing device for a camera and more particularly to such an automatic focusing device in which the detection of focus is carried out by measuring the light transmitted through the photographing lens of the camera. The present invention relates also to a photographing lens for use in automatic focusing cameras.
2. Description of the Prior Art
As an automatic focusing device for single-lens reflex camera there is well known such a focus detection device in which the existing deviation of the real image plane of the object from a determined focal plane of the taking lens such as the film plane is detected by measuring the light transmitted through the taking lens. Typical examples of such focus detection device are disclosed, for example, in U.S. Pat. Nos. 4,185,191 and 4,264,810. With this type of focus detection device, the taking lens of the camera can be brought into the in-focus position by driving the focusing lens of the taking lens system through a distance corresponding to the detected image plane deviation.
Where the taking lens has a definite constant focal length, the relationship between the detected deviation and the distance over which the focusing lens must be shifted remains substantially constant. Therefore, in this case, the necessary high precision control for focusing can be performed in a relatively simple manner by shifting the focusing lens correspondingly to the detected deviation of the image plane.
However, where the taking lens is variable in focal length as in the case of zoom lens, the above relationship between detected deviation of image plane and the amount of shift of the focusing lens is no longer constant but variable. Therefore, in this case, it is very difficult to attain high precision and rapid automatic control of focusing. In addition, for such automatic focus control used in a single-lens reflex camera, it is absolutely necessary to transmit various signals in a reliable manner between the exchangeable taking lens structure unit and the camera body. In these years, the number of signals to be transmitted to the camera body from the exchangeable lens structure unit has increased more and more. For example, various information of the taking lens such as preset or controlled aperture value, object distance and maximum aperture value are desired to be transmitted to the camera body as electrical signals.
Naturally this means that many terminals must be provided to transmi these various electric signals to the camera body from the exchangeable lens unit. However, the space in the lens unit available for such terminals is very limited. It is very difficult in practice to reserve a sufficient space to provide such a large number of terminals. Further, the increase in number of such signal transmission terminals may cause many serious troubles such as wrong electrical contact.